This invention relates to a composite changeover-type reaction power plant which can be selectively operated as a turbofan engine and a ramjet engine utilizing a common annular bypass ramjet air passage duct.
Historically aircraft have been developed to give increasingly high performance and speeds reaching well into the hypersonic regime. Currently available propulsion systems are incapable of optimally serving the sweeping performance and speed ranges. Propeller engines, for instance, have large mass flows which make them desirable for short take-off and landing distances, and they give good efficiency at relatively slow airspeeds and in relatively low performance ranges, whereas pure jet engines give high thrust performance with high specific thrust levels and ensure relatively high air speeds at satisfactory efficiency. Maximum flight speeds in the hypersonic range, however, cannot be achieved with airbreathing solutions, except for ramjets whose specific thrusts and power weight ratios are extremely high, but whose efficiencies are substantially poorer especially at relatively low airspeeds. For physical and technical reasons it is impossible to have a single engine type optimally satisfy all flight requirements from takeoff to satisfactory climb and cruise speeds all the way to high and ultrahigh speeds.
This called for the development of composite changeover-type powerplants which, as described for the known experimental "Griffon" aircraft (Flug Revue International, January 1967, p. 15), comprise a central turbojet engine and enveloping it, a supersonic-intake ramjet engine, where the turbojet engine is used for take-off and landing flight conditions and for the low-speed range, while the ramjet engine is used to achieve high air speeds. In an attempt to achieve still improved conditions at take-off and landing conditions and also at low airspeeds, published engine studies like that of the above-mentioned literature reference illustrated in FIG. 3 use a turbofan or bypass engine arranged centrally inside a ramjet engine, where use is made of a low-pressure front fan engine to provide the second cold cycle, said front-fan engine having corotating compressor stages and being driven by a low-pressure turbine of the core engine's compressor drive turbine. Bypass engines provide an advantage in that they augment the air or mass flow. The turbofan nevertheless is adversely affected also by disadvantages when deactivated during ramjet operation, i.e., with a fan of conventional construction having corotational compressor stages and intervening fixed stator cascades, appreciable power losses are expected even when extensive use is made of variable-geometry stator cascades when on the one hand, effective use shall be made of the flow capacity of the annulus in the fan area and when on the other hand, high pressure losses caused by flow separation in the various cascades shall be avoided. This is because in fans of conventional construction, there is a close relationship between fan speed and mass flow, so that by the same token, also power absorption can be influenced within narrow limits only.
Fans of conventional construction, even when fitted with variable-geometry banes, are also characterized by the fact that at the reduced speeds N/.sqroot.T, which are lowered at high flight mach numbers, especially in the hypersonic region, despite maximum circumferential speeds, the level of the axial mach numbers and, thus, also the reduced flow ##EQU1## decreases. Since the mach numbers at the compressor exit and the afterburner inlet show a fixed interrelationship, the compressor in the ramjet engine represents an extreme obstacle to the full utilization of the available flow area in the afterburner in the interest of maximum allowable mach numbers or maximum flow.
The disadvantages inherent in previously disclosed versions of composite powerplants combining turbofans with ramjets can be summarized such that, one, fan engines of conventional construction produce no satisfactory engine powers and efficiencies at high supersonic air speeds and that, two, these fan engines, when shut down during ramjet operation, constitute a notable obstacle to the flow that prevents the full utilization of the available flow area in the ramjet cycle in the interest of maximum allowable flow mach numbers and maximum flow.
In a broad aspect of the present invention the disadvantages affecting composite powerplants comprising of a turbofan engine and a ramjet engine, with conventional fans, are eliminated and measures are proposed to conceptually improve such composite engines such that at the same motor conditions or the same design values and conditions for both individual engines, power and efficiency can be improved.
It is a particular object of the present invention to provide a composite powerplant of the general type described above with the inlet compressor stages formed of counterrotating blade stages without intervening fixed stator vanes. A fundamental advantage provided by a fan with counterrotating statorless rotors and with fan blades capable of rotation about their longitudinal axis, is that at axial inlet flow and simultaneously axial outlet flow the blades of the 1st and the 2nd rotors can be positionally varied such that at a certain speed, pressure ratio and flow can be detached one from the other to a considerable degree (cf. FIG. 9). This amount of flexibility can additionally be improved by the use of a variable inlet stator cascade upstream of the first compressor rotor, which also serves to improve afflux conditions for both rotor stages in the interest of optimum fan efficiency and functioning.
An advantage obtained in this manner over conventional engines in that the composite powerplant arranged in accordance with the present invention enables acceptable noise levels to be achieved during take-off and climb. In this respect it is assumed in the interest of noise attenuation that take-off and initial climb to about 4 km in altitude at mach numbers around 0.6 will be in the dry turbofan configuration, with the fan pressure ratio being limited to values around 2.2 to 2.4. But when the noise attenuation requirement is obviated--e.g. when climbing to higher altitudes--the engine geometry can be adjusted for a high fan pressure ratio in the 3.5 to 4.0 range to achieve high specific thrust in dry or heated operation at favorable specific fuel consumption.
When the fan blades are feathered during ramjet operation, their drag and the pressure drop in that area is appreciably reduced. Another special potential for optimizing flow conditions in the ramjet rests in the fact that the core engine practically lies in the cross-sectional shade of the fan engine hub. This is again benefited by the fact that counter-rotating statorless compressors generally have their optimum efficiency at greater deliveries than do conventional compressors, i.e., at given peripheral speeds in the range of greater axial velocities, so that compared with conventional compressors, greater axial flow densities or greater flows at a given area are achieved at aerodynamically and/or mechanically allowable peripheral speeds. Also, the variable fan blades make it possible to vary the axial velocities over a wide range while maintaining the zero-swirl reflux, so that especially at off-design load, i.e. at reduced peripheral speeds, the axial fluid density does not not necessarily drop as heavily as with conventional compressors. Also, the high air flow necessitated by the feathered pitch of the fan blades particularly above the changeover point, i.e., in the mach 2.5 to 3.0 range, makes for lower specific thrust at a given thrust requirement and a given cross-sectional engine area, i.e., in conjunction with adequate reheating at lower nozzle temperatures, for substantially lower fuel consumption.
A further advantage afforded by the fan engine arranged in accordance with the present invention is that in hypersonic ramjet operation, i.e., also above the above-mentioned changeover point, the same mach number level can conceptually be reached as in subsonic operation, i.e., maximally 0.2 to 0.3 at the flameholder inlet, while when using a conventional fan the mach number level would be restricted to about 0.10 with accordingly low flow level.
The aerodynamic design of a fan engine with the usual pressure ratio around 2.5 to 3.5 requires considerable constriction of the flow passage towards the fan exit, so that this produces a throat, as it were, which may well be favorable for fan operation, but adversely affects ramjet operation. Viewed in more detail the problem presents itself thus: conceptually the composite power plant of the present invention with its fixed duct structure in the area of the fan and the downstream flow passage with afterburner and variable convergent-divergent nozzle, can effectively be operated also as a ramjet engine, if the flow areas are attuned such that in turbofan operation, favorable mach numbers are achieved at the fan inlet and exit and at the flameholder inlet. In this context, mach numbers around 0.7 at the fan inlet, 0.5 to 0.6 at the fan exit and 0.2 at the flameholder inlet, e.g., can be considered normal values. It may prove favorable, however,--especially in LH.sub.2 --based ramjet operation--to increase thrust by maximizing flow in the high mach number regime above the changeover point, i.e. at high inlet temperatures with correspondingly moderate temperature rise in the afterburner, such that a mach number in the 0.3 to 0.4 range is allowed at the flameholder inlet. In this case the mach numbers at the exit of the deactivated fan with fixed outer contour would rise tremendously, so that high pressure losses would ensue in the downstream flow passage.
If in this case, i.e., above the changeover point, it is intended to optimize the overall system comprising the intake, flow passage between fan inlet and flameholder inlet, after-burner and nozzle towards maximum flow or thrust, it is proposed in accordance with preferred embodiment of the present invention that the engine outer casing is designed such that when the composite power plant is operated in the ramjet mode, the cross-sectional areas of the flow passage at the fan exit can be greatly widened.
Blocking the core engine at its air intake and nozzle to seal it from the flow passage of the ramjet engine--as it is intended in accordance with the present invention during ramjet operation--serves to protect the deactivated core engine and fan plus associated blades from overheating by hot gases penetrating from the ramjet duct. This produces--with respect to the core engine plus fan, a virtually closed space which at higher flight mach numbers is internally swept by a cooling gas of a suitable temperature and of a pressure that is slightly higher than that of the externally located flow passage of the ramjet engine. The effectiveness of environmental control provisions for the core engine plus fan naturally depends on what the bearing lubrication concept looks like, which materials are used, and which coolant is employed. It should in any case be possible--up to mach numbers around 2.2 to 2.4--to make do with a conventional air system that does not require artificial cooling of the cooling air to be tapped from the engine, especially so as bearing chamber temperatures of about 250.degree. C. are considered allowable. At higher mach numbers, which at once correspond to ramjet operations, air may still be the proper coolant, but will have to be cooled down to allowable temperature in a chiller. Since the amounts of air needed for environmental control of the core engine plus fan--enough to neutralize the heat input from the flow around the core engine--are relatively small, the cooling effort will remain in acceptable limits. Conceptually, there may naturally be alternative cooling systems that may lend themselves--e.g. when LH.sub.2 is used --to use for the present concept of counterrotating variable fan in ramjet operation with sealed core engine as here described.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.